Field of the Invention
The invention concerns a method to acquire magnetic resonance data with a diffusion-weighted magnetic resonance sequence using a magnetic resonance apparatus with a gradient coil arrangement that has three gradient coils, each designed to generate a gradient in a gradient direction, the gradient directions being orthogonal to one another. The invention also concerns a magnetic resonance apparatus to implement such a method.
Description of the Prior Art
Magnetic resonance imaging is a widely known and prevalent imaging method that is often applied in a medical context. Within the scope of a magnetic resonance sequence, nuclear spins in a subject are excited by at least one excitation pulse and the resulting magnetic resonance signals generated by the spins are acquired in a readout time period after a defined duration (what is known as the echo time). A spatial coding takes place with the use of gradients that are switched (activated) at different points in time of the magnetic resonance sequence. A differentiation is typically made between a gradient known as: the slice selection gradient that defines the slice to be excited; and the phase coding gradient (typically orthogonal to this slice selection gradient) that codes a position in a direction within the slice by the phase of the signal; and the readout gradient that allows a sweeping of the slice in a direction that is orthogonal to the phase coding gradient and the slice selection gradient. Overall, within the slice gradients produce a trajectory or path within k-space, along which data representing the signals are entered into k-space. The measured k-space data (magnetic resonance data) can be translated into the spatial domain by a Fourier transformation, so that magnetic resonance images are created. In modern magnetic resonance devices, radio-frequency coils are used to generate excitation pulses as well as for the reception of the magnetic resonance signals. For the generation of the gradient pulses a gradient coil arrangement is provided that typically has three gradient coils that each correspond to a different spatial direction in an imaging region, i.e. can generate a gradient of the magnetic field in this gradient direction. Many sequences use the z-direction as the slice selection direction, the y-direction as the phase coding direction, and the x-direction as the readout direction.
In many magnetic resonance examinations, sequences known as diffusion-weighted magnetic resonance sequences are used, particularly for magnetic resonance acquisitions of the head of a patient. Diffusion-weighted magnetic resonance sequences have also been suggested for breast examinations. Diffusion-weighted magnetic resonance sequences are characterized by additional gradient pulses (known as diffusion gradients) that can be assembled as a diffusion module, being integrated into the magnetic resonance sequence. Most often, EPI (echoplanar imaging) readout trains are combined with an upstream diffusion module. The best image results are thereby achieved when the echo times (TE) are as short as possible. For example, at clinical resolutions, a sequence known as the RESOLVE sequence thus allows echo times of approximately 60 ms. The RESOLVE sequence was proposed in an article by D. A. Porter and R. M. Heidemann, “High resolution diffusion-weighted imaging using readout-segmented echo-planar imaging, parallel imaging and a two-dimensional navigator-based reacquisition”, Magn. Reson. Med. 62:468-475 (2009).
Many diffusion-weighted magnetic resonance sequences measure different slices in a target object to be acquired, which is also designated as two-dimensional magnetic resonance imaging. Techniques are often also used that allow multiple slices to be acquired simultaneously in order to reduce the repetition time (TR) and the entire acquisition time. In particular, modified radio-frequency pulses can be used in order to excite and refocus the magnetization of multiple different slices in an actually simultaneous manner. The resulting echoes then similarly arise simultaneously, wherein the superimposed individual signals of the different slices that are then sampled as a magnetic resonance signal can be divided algorithmically, for example by the use of spatially dependent information of multiple reception coils. This is an application of what is known as parallel imaging, for which one known embodiment that is often used is the GRAPPA technique, as described in the article by M. A. Griswold et al., “Generalized autocalibrating partially parallel acquisitions (GRAPPA)”, Magn. Reson. Med. 47(6):1202-1210 (2002). One extension of the fundamental idea of the simultaneous excitation and refocusing is known as the CAIPIRINHA method, as described in F. A. Breuer et al., “Controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) for multi-slice imaging”, Magn. Reson. Med. 53:684-691 (2005). For example, it has been proposed to combine a CAIPIRINHA method using short gradient pulses (“blipped”) for simultaneous imaging of multiple slices with the diffusion-weighted, readout-segmented echoplanar imaging sequence (rs-EPI—readout segmented echo planar imaging), as described in U.S. Pat. No. 7,205,763 and the aforementioned article by D. A. Porter et al.
These references also note that navigators can be used for real-time feedback. The acquisition of the navigators then forms a second partial sequence of the magnetic resonance sequence, which concurrently uses the excitation signal emitted in the first partial sequence for acquisition of the actual magnetic resonance data and uses a new refocusing pulse and a new readout time period. A navigator feedback is typically used in connection with the rs-EPI sequence or other diffusion-weighted magnetic resonance sequences in order to identify and re-measure readout segments with strong, movement-induced phase errors or other movement effects, if they cannot be reliably corrected by a correction technique (for example phase correction) under consideration of the two-dimensional navigators.
EPI sequences and a few other magnetic resonance sequences used within the scope of the diffusion-weighted imaging use sinusoidal readout gradients in the readout direction in the readout train, while short gradient pulses—known as “blips”—are switched in the phase coding direction between the readout processes. With regard to the k-space trajectory, jumps from one k-space line to another k-space line that should be read out can be made as quickly as possible via these “blips”.
In such an embodiment, diffusion-weighted magnetic resonance sequences consequently place extremely high demands on the gradient coil arrangements of the magnetic resonance devices, among other things on the available slew rates (SR). In order to achieve short echo times (TE), extremely high slew rates of >175 T/m/s on a gradient axis are used, which is possible only with high-end gradient systems. Accordingly, it is extremely difficult to execute a diffusion-weighted magnetic resonance sequence supplying evaluable results at a low-end system with a weaker gradient system allowing only low slew rates. However, diffusion-weighted magnetic resonance sequences and the corresponding clinical results are also required at such magnetic resonance devices.
To solve this problem, given the use of low-end magnetic resonance devices it is known to only temporally extend the known diffusion-weighted magnetic resonance sequences so that they can be executed with the present specifications of the gradient coil arrangement. This entails a steep rise of the echo times, for example from 67 ms to 121 ms given known clinical magnetic resonance sequences. The primary cause for this extension of the echo times is that the readout train must be markedly extended due to the markedly lower slew rates.